1. Field of the Invention
The present invention relates to an image formation apparatus which performs the image area separation of a read image to provide character and photograph areas and forms an image based on the read image, an image formation apparatus control method, and a storage medium for storing a computer-readable program.
2. Description of Related Art
An image area separation method of judging, from an image signal in the predetermined area of a read image, whether the inside of the predetermined area is entirely a character area or a photograph area is present as a conventional art.
Moreover, as image area separation means of judging whether the predetermined area is a character area or a photograph area as described above, a method of performing a discrete Fourier transform in the predetermined area and perform judgment by the amount of frequency components is also known as the conventional art.
Furthermore, means for switching image formation means during image output with respect to the area subjected to the image area separation is also known as the conventional art.
An image separation judgment processing in the conventional image formation apparatus will concretely be described hereinafter with reference to FIGS. 15 and 16.
FIG. 15 is a schematic diagram showing a small area which is considered to be a character area or a photograph area in the conventional image formation apparatus, and the small area corresponds to a small area from which the image signal of 8 pixels×8 pixels is extracted in the read image.
The conventional image formation apparatus performs frequency conversion such as a fast Fourier transform (FFT) in the small area as shown in FIG. 15, and performs the image area separation to judge whether the small area is a character area or a photograph area.
FIGS. 16A and 16B are characteristic diagrams showing the frequency characteristic when the read image is subjected to the frequency conversion in the conventional image formation apparatus, FIG. 16A is a one-dimensional graph showing the frequency conversion result of a character image, and FIG. 16B is a one-dimensional graph showing the frequency conversion result of a photograph image. Additionally, in both FIGS. 16A and 16B, the abscissa indicates frequencies, and the ordinate indicates frequency components.
The conventional image formation apparatus judges that the area is a character area when the frequency component in the vicinity of the Nyquist frequency of reading resolution is high as shown in FIG. 16A, and judges that the area is a photograph area when the frequency component is low as shown in FIG. 16B. Additionally, the character area includes a mesh point image.
In the image area separation judgment method of the conventional image formation apparatus of judging, from the image signal in the predetermined area of the read image, whether the entire area shown in FIG. 15 is a character area or a photograph area, since the processing is performed by an area unit, there is no interaction between adjacent areas, and the judgment easily results in rapid area judgment.
For example, when the read image of 600 dpi×600 dpi (both the ordinate (Y direction of FIG. 15) and abscissa (X direction of FIG. 15) indicate 600 dpi) is subjected to the image area separation with the small area of 8 pixels×8 pixels as shown in FIG. 15, even the small area is considered to be a character area or a photograph area for each area of about 340 μm×340 μm.
However, when an output is emitted through the image formation apparatus, the size of 340 μm×340 μm (small area of (8×8) pixel unit shown in FIG. 15) can be confirmed with the naked eye as an apparent size. Actually the size of about several hundreds of micrometers can easily be distinguished with the naked eye, and the image change of this size is securely recognized as an unfavorable false outline or a nonuniformity.
Therefore, when the technique of switching the image formation means in the character area and photograph area based on the judgment result of the small area as shown in FIG. 15 is introduced, and when a difference is seen in the output characteristic between the areas, the unfavorable outline is recognized to such an extent that it is apparently visible with the naked eye.
This will concretely be described hereinafter with reference to FIGS. 17 and 18.
FIG. 17 is a schematic diagram showing the boundary between the character area and the photograph area when the image area separation is performed for each small area shown in FIG. 15 in the conventional image formation apparatus.
In FIG. 17, T denotes the character area, and P denotes the photograph area. Numeral 1401 denotes the boundary between the character area T and the photograph area P.
FIG. 18 is a schematic diagram showing the image formation result when the character area is adjacent to the photograph area and the image formation means is changed in the character and photograph areas in the conventional image formation apparatus, and the same components as those of FIG. 17 are denoted with the same reference numerals.
FIG. 18 shows the output result of the image formation apparatus in which toner is visualized as the image in a black part. In the character area T, black/white is clearly outputted to attach importance to sharpness rather than to gradation, and in the photograph area P, no steep density difference is made between black and white with due consideration to attach importance to gradation.
Therefore, when the image area is considered to be the mixture of the character area T and the photograph area P (FIG. 17), as shown in FIG. 18, in the character area T, the image formation is performed by attaching importance to the character sharpness, without laying any emphasis on the gradation. On the other hand, in the photograph area P the image formation is performed by laying emphasis on the gradation rather than the sharpness. As a result, the boundary 1401 between the character area T and the photograph area P becomes conspicuous, and there is a problem that the unfavorable images such as the false outline are formed.
In this case, when the difference of the image formation means is minimized between the character area and the photograph area, the result becomes satisfactory, but another problem occurs that it is difficult to set the sharpness demanded for the character to be compatible with the gradation demanded for the photograph.
Moreover, in the image formed by subtly combining the character area and photograph area, for example, in the character image on the photograph, the image is considered to be either the character area or the photograph area by the image area separation technique of the conventional image formation apparatus in which only two types of judgments can be performed in the predetermined area. Therefore, when the character image and photograph image are present in the small area as in the character image on the photograph, there is also a problem that the image of either the photograph or the character is sacrificed.
Furthermore, as the means of judging whether the predetermined area is the character area or the photograph area, in the conventional method of performing the discrete Fourier transform in the predetermined area and performing the judgment by the amount of the frequency component, a load of hardware or software is inevitably increased. This is because the discrete Fourier transform is relatively complicated as the calculation of the image signal.
Therefore, in the conventional image formation apparatus in which the discrete Fourier transform is performed in the above-described predetermined area and the judgment is performed by the amount of frequency components, consideration also needs to be given to processing speed, and there is also a problem that an expensive processor, and the like have to be mounted.